80 ROBERT STANLEY McEWEN 
negative geotropism would show the latter more negatively 
geotropic than the former. This would result from the fact 
that though equally impelled to move upward, the phototropie 
animals would be constantly handicapped by the attraction of 
the light from below. In order to remedy this difficulty, there- 
fore, three lamps of the same candle power were arranged in a 
vertical line, so that one came opposite the foot of the tube, one 
opposite the center, and one opposite the top. The end of each 
bulb was a distance of 41 cm. from the tube. The latter, more- 
over, was now held in its vertical position by a wire support, so 
that there was no danger of its wabbling. With these improve- 
ments, the following experiments were undertaken. 
In the first place, it was decided to run a test on some wild 
flies in order to get some data which should be comparable with 
that obtained by the same system for the mutants. Also in mak- 
ing these tests it was decided incidentally to run a few checks 
on the effect of wing removal, in order to make sure that the 
former tests were not invalidated by the position of the single 
lamp. For this purpose three groups of insects, each contain- 
ing ten males and ten females, were selected, and kept in the 
usual manner until 5 days old. They were then tested as de- 
scribed above. After the test, the males in the groups which 
we shall designate as A and B were etherized and in the case of 
group B the wings were removed. Eight hours later both groups 
were retested. The control males in Group A were now also 
operated on, and tested for the third time 10 hours later. In 
the case of the females, Group A was operated on after the first 
test and Group B kept as a control. Twenty-four hours later 
both these female groups were tested again. Group B wasnow 
operated on and tested after eight hours. The results of these 
tests are summarized below (table 14). 
This experiment confirms the conclusions already set forth re- 
garding the effect of wing removal. In other words, the re- 
arrangement of the lights has produced no significant effect. The 
only thing of note in the record is the extremely high gravity index 
registered by the males of Group B after being operated on. To 
determine whether this is of any significance or not will require 
